A8431. White LED Driver Constant Current Step-up Converter

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1 Features and Benefits Output voltage up to 32 V ( level) 2. to 0 V input Drives up to 4 LEDs at 20 ma from a 2. V supply Drives up to LEDs at 20 ma from a 3 V supply.2 MHz switching frequency 300 ma switch current limit μa shutdown current pin eliminates the need for an external Zener diode on the output Package: -pin MLP/DFN (suffix EH) Approximate scale Description The A843 is a noninverting boost DC-DC converter that provides a programmable constant-current output up to 32 V for driving white LEDs in series.the A843 also offers an (overvoltage protection) pin. Driving the LEDs in series ensures identical currents and uniform brightness. Up to four white LEDs can be driven at 20 ma from a single cell Li-ion or a multicell NiMH power source. Up to two parallel strings of eight white LEDs can be driven at 20 ma by increasing the supply voltage up to 0 V. The A843 incorporates a power switch and a feedback sense amplifier to provide a solution with minimum external components. The output current can be set by adjusting a single external sense resistor and can be varied with a voltage or a filtered PWM signal when dimming control is required. The high switching frequency of.2 MHz allows the use of small inductor and capacitor values. The A843 is provided in a 0.7 mm nominal height, -pin, 2 mm 3 mm MLP package. It is lead (Pb) free, with 00% matte tin leadframe plating. Applications include: LED backlights Portable battery-powered equipment Cellular phones PDAs (Personal Digital Assistant) Camcorders, personal stereos, MP3 players, cameras Mobile GPS systems Functional Block Diagram FB V REF.2 V 9 mv A R C C C A2 R S Q Driver Σ Ramp Generator EN.2 MHz Oscillator GND 28.30

2 Selection Guide Part Number Packing* A843EEHTR-T 00 pieces per 7-in. reel *Contact Allegro for additional packing options Absolute Maximum Ratings Characteristic Symbol Notes Rating Units Pin Voltage V 0.3 to 3 V Pin Voltage V 0.3 to 3 V Remaining Pin Voltage 0.3 to 0 V Operating Ambient Temperature T A Range E 40 to 8 ºC Maximum Junction Temperature T J (max) 0 ºC Storage Temperature T stg to 0 ºC Pin-out Diagram GND 2 FB 3 4 EN Terminal List Table Pin Name Function Internal power FET 2 GND Ground 3 FB Feedback input 4 EN input Overvoltage protection Input supply R θja = 0 C/W, measured with 4-layer PCB. Please refer to application note Package Thermal Characteristics, for thermal performance measurement for 2 x 3 mm MLP package for additional information. Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

3 ELECTRICAL CHARACTERISTICS at T A = 2 C, V IN = 3 V (unless otherwise noted) Characteristics Symbol Test Conditions Min. Typ. Max. Units Input Voltage Range V IN 2. 0 V Active ma Supply Current I SUP Shutdown (EN = 0 V) 0. μa Feedback Reference Voltage V FB mv Feedback Input Current I FB 20 7 na Switch Current Limit I LIM 300 ma Switch Frequency F MHz Switch Maximum Duty Cycle D 8 90 % Switch Saturation Voltage V CE(SAT) 30 mv Switch Leakage Current I SL V = V μa Input Input Threshold Low V IL 0.4 V Input Threshold High V IH. V Input Leakage Leakage I IL μa Overvoltage Protection Output Overvoltage Rising Limit V R V Output Overvoltage Falling Limit V F V Output Overvoltage Hysteresis V HYS 0. V Pin Resistance R.0 MΩ Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

4 Operating Characteristics Using Typical Application Circuit (Schematic ) Quiescent Current versus Input Voltage Quiescent Current versus Temperature Quiescent Current (ma) Quiescent Current (ma) V IN (V) Temperature ( C) Feedback Bias Current versus Temperature Switching Frequency versus Temperature 20.2 Feedback Bias Current (na) 0 Switching Frequency (MHz) Temperature ( C) Temperature ( C) 300 Switch Pin Voltage versus Temperature Conversion Efficiency versus Input Voltage 9 V CE(SAT) (mv) Conversion Efficiency (%) LEDs 4 LEDs LEDs Temperature ( C) V IN (V) Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

5 Functional Description Typical Application A typical application circuit for the A843 is provided in schematic diagram. This illustrates a method of driving three white LEDs in series. The conversion efficiency of this configuration is shown in chart. Pin Functions The diagram also shows a method of connecting the individual pins, whos functions are described as follows:. Supply to the control circuit. A bypass capacitor, C, must be connected from close to this pin to GND.. Low-side switch connection between the inductor, L, and ground. Because rapid changes of current occur at this pin, the traces on the PCB that are connected to this pin should be minimized. In addition, L and the diode D should be connected as close to this pin as possible.. Overvoltage Protection sense pin to protect the A843 from excessive voltage on the pin. This pin should be connected to the output capacitor, C2. To disable this feature connect the pin to ground. EN. Setting lower than 0.4 V disables the A843 and puts the control circuit into the low-power Sleep mode. Greater than. V fully enables the A843. GND. Ground reference connected directly to the ground plane. The sense resistor, R, should have a separate connection directly to this point. FB. Feedback pin for LED current control. The reference voltage is 9 mv. The top of R is typically connected here. L 22 μh D 90 Conversion Efficiency versus Current 8 Li-ion 2.V to 4.2V C.0 μf A843 EN GND FB R.3Ω C μf Efficiency (%) V IN = 3 V V IN = 4 V LED Current (ma) Schematic. Typical application Chart. Conversion efficiency when driving three LEDs in the typical application circuit. Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

6 Device Operation The A843 uses a constant-frequency, current-mode control scheme to regulate the current through the load. The load current produces a voltage across the external sense resistor (R in schematic ) and the input at the FB pin. This voltage is then compared to the internal 9 mv reference to produce an error signal. The switch current is sensed by the internal sense resistor and compared to the load current error signal. As the load current increases, the error signal diminishes, reducing the maximum switch current and thus the current delivered to the load. As the load current decreases, the error signal rises, increasing the maximum switch current and thus increasing the current delivered to the load. To set the load current, ensure that the required internal reference value of 9 mv is produced at the desired load. To do so, select a resistance value for the sense resistor, R (Ω), such that: R = 9 mv I LOAD where I LOAD is the target load current (ma). The table below shows typical values for R. Note that the resistance value is from the standard E9 series. As load current is reduced, the energy required in the inductor, L, diminishes, resulting in the inductor current dropping to 0 A for low load-current levels. This is known as Discontinuous mode operation, and results in some lowfrequency ripple. The average load current, however, remains regulated down to 0 A. In Discontinuous mode, when the inductor current drops to 0 A, the voltage at the pin rings, due to the capacitance in the resonant LC circuit formed by the inductor and the capacitance of the switch and the diode. This ringing is low-frequency and is not harmful. It can be damped with a resistor across the inductor, but this reduces efficiency and is not recommended. Overvoltage Protection An overvoltage event can occur when the LEDs become disconnected or fail in an open state. In these cases, the current flow through the sense resistor, R, becomes 0 A and thus the feedback voltage, V FB becomes 0 V. The A843 compensates by increasing the on time of the switch, which increases the output voltage. The A843 has built-in protection to guard against excessive voltage on the pin. If the output voltage exceeds the typical level of the Output Overvoltage Rising Limit, V R, then the overvoltage protection circuitry shuts off the internal switch until the output voltage falls below the Output Overvoltage Falling Limit, V F. At this point, the A843 operates normally. There is no need for an external Zener diode for the A843. Power Dissipation versus I OUT PD (mw) IOUT (ma) Target Load Current (I LOAD ) (ma) Sense Resistor (R) (Ω) Vin = 3V, 3 LED Vin = V, 3 LED Vin = 3V, 4 LED Vin = V, 4 LED Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

7 Application Information Component Selection The component values shown in schematic are sufficient for most applications. To reduce the output ripple, L may be increased, but in most cases this results in excessive board area and cost. Inductor Selection. With an internal PWM frequency of.2 MHz, the optimal L value for most cases is 22 μh. For worst case conditions (high output voltage and current and low input voltage), the inductor should be rated at the switch current limit, I LIM. If high temperature operation is required, a derating factor will have to be considered. In some cases, where lower inductor currents are expected, the current rating can be decreased. Several inductor manufacturers, including: Coilcraft, Murata, Panasonic, Sumida, Taiyo Yuden, and TDK, have and are developing suitable small-size inductors. Diode Selection. The diode should have a low forward voltage to reduce conduction losses. In addition, it should have a low capacitance to reduce switching losses. Schottky diodes can provide both these features, if carefully selected. The forward voltage drop is a natural advantage for Schottky diodes, and it reduces as the current rating increases. However, as the current rating increases, the diode capacitance also increases. As a result, the optimal selection is usually the lowest current rating above the circuit maximum. In this application, a current rating in the range from 00 ma to 200 ma is usually sufficient. Capacitor Selection. Because the capacitor values are low, ceramic capacitors are the best choice for this application. To reduce performance variation as temperature changes, low- drift capacitor types, such as X7R and XR, should be used. A.0 μf capacitor on the pin is suitable for most applications. In cases where large inductor currents are switched, a larger capacitor may be needed. The output capacitor, C2, can be as small as 0.22 μf for most applications and most input/output voltage ranges. Increasing the capacitor value on the output aids in increasing the efficiency of low input voltage/high output voltage conditions. Suitable capacitors are available from TDK, Taiyo Yuden, Murata, Kemet, and AVX. Dimming Control LED brightness can be controlled either: (a) by modifying the voltage at the top of R to control the LED current, I LOAD, directly, or (b) by using a PWM signal on the EN pin to chop the output. Feedback Modulation. By adding a voltage drop between the FB pin and R, as shown in schematic 2, the LED current, I LOAD, can be made to decrease. As V C (control voltage) increases, the voltage drop across R2 also increases. This causes the voltage at FB to increase, and the A843 reduces I LOAD to compensate. As V C increases further, the current drops to 0 A, and R2 maintains the full 9 mv on FB. Reducing V C diminishes the voltage across R2 until, when V C is at 9 mv, there is no drop across R2 and the current level is defined by R. Reducing V C below 9 mv causes I LOAD to increase further, due to the voltage drop across R2 in the reverse direction. This continues until, when V C is at 0 V, there is approximately mv across R2. At that point, I LOAD (ma), is defined as: I LOAD = 00 mv R where R is the resistance of the sense resistor (Ω). Li-ion 2. V to 4.2 V C.0 µf L 22 µh D A843 EN GND FB 4 V C 2 3 R3 90 kω R2 kω R.3 Ω C µf Schematic 2. Dimming control with dc voltage Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

8 PWM Control. The control voltage, V C, also can be generated by a filtered PWM signal, as shown in schematic 3. In this case, a 0% duty cycle (PWM = 0 V) corresponds to full brightness and a 00% duty cycle causes the LED current, I LOAD, to go to 0 A. By applying a PWM signal directly to the EN pin, the A843 is turned on or off, and I LOAD is either full (as defined by R) or 0 A. By varying the duty cycle of the PWM signal, the LED brightness can be controlled from off (0% duty cycle) to full (00% duty cycle). The PWM frequency should be in the range from khz to 0 khz. Several other schemes are possible, for example, digitally switching additional resistors across R to increase I LOAD. In this case, R would be selected for the minimum desired brightness. Li-ion 2. V to 4.2 V C.0 µf V C (PMW) L 22 µh D A843 EN GND FB R4 0 kω 2 3 R3 90 kω C3 00 nf R2 kω Schematic 3. Dimming control with filtered PWM 4 R.3 Ω C µf Start-Up To provide fast start-up operation, no soft start is implemented in the control circuit. At power-on, the bypass capacitor, C, is discharged, which means that the supply must provide the in-rush current through the inductor, L. This can be reduced by modulating the feedback with a soft-start circuit as shown in schematic 4. When power is first applied, the capacitor C3 is discharged and pulls the FB pin high, reducing the output drive to minimum. As C3 charges, when the bottom drops below about 0.8 V, the feedback from the sense resistor, R, takes over full control of the output current. Li-ion 2.V to 4.2V C.0 µf L 22 µh D A843 EN GND FB 2 3 Schematic 4. Soft start operation 4 C3 2.2 nf R2 kω R3 kω R.3 Ω C µf Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

9 Parallel LED Strings The A843 can be used to power parallel strings of LEDs, which have the same number of LEDs on each string. It is important that the voltage drop is the same across all of the parallel strings, to ensure that all of the LEDs are illuminated and that the current though each string is equal. A typical circuit with two parallel strings is shown in schematic. The coversion efficiency of this configuration is shown in chart 2. Li-ion 2. V to 4.2 V C.0 µf L 22 µh D A843 EN GND FB R.3 Ω R2.3 Ω C µf Schematic. Parallel strings of LEDs Conversion Efficiency for Two Parallel Strings 9 90 Efficiency (%) Two 3-LED strings Two 4-LED strings Two 7-LED strings Input Voltage (V) Chart 2. Conversion efficiency when driving two parallel strings of varying lengths Northeast Cutoff, Box 03 Worcester, Massachusetts (08)

10 Package EH, -pin MLP/DFN A C PCB Layout Reference View B. A All dimensions nominal, not for tooling use (similar to JEDEC MO-229WCED-) Dimensions in millimeters Exact case and lead configuration at supplier discretion within limits shown Terminal # mark area B Exposed thermal pad (reference only, terminal # identifier appearance at supplier discretion). C Reference land pattern layout (based on IPC73) All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary to meet application process requirements and PCB layout tolerances; when mounting on a multilayer PCB, thermal vias at the exposed thermal pad land can improve thermal dissipation (reference EIA/JEDEC Standard JESD-) Copyright 2003, 2007, The products described here are manufactured under one or more U.S. patents or U.S. patents pending. reserves the right to make, from time to time, such de par tures from the detail spec i fi ca tions as may be required to permit improvements in the per for mance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the failure of that life support device or system, or to affect the safety or effectiveness of that device or system. The in for ma tion in clud ed herein is believed to be ac cu rate and reliable. How ev er, assumes no responsibility for its use; nor for any in fringe ment of patents or other rights of third parties which may result from its use. For the latest version of this document, visit our website: Northeast Cutoff, Box 03 Worcester, Massachusetts (08)